This invention relates to improvements in W-N gears or gears of the kind whose basic rack tooth profile on normal or transverse reference plane or the tooth surface is composed of addendum-defining convex circular arcs and dedendum-defining concave circular arcs and is intended to provide novel double tooth contact or double mesh type gears of the kind described which are highly advantageous from the viewpoints of manufacture and performance.
As is well known in the art, a pair of mating W-N gears of the kind described operate with their circular arc tooth surfaces successively making meshing contact and, having a transverse contact ratio of zero or approximately zero, rely for transmission of rotation exclusively upon the tooth trace. In this sense, W-N gears in essence can be considered helical gears.
With conventional forms of W-N gear, meshing contact of the addendum and that of the dedendum take place at the same time and separately in different reference planes, their location shifting progressively in the direction of tooth trace with rotation of the mating gears. Accordingly, with W-N gears, transmission of rotation can be effected smoothly so long as a requisite value of axial overlap contact ratio is obtained.
Generally, with involute spur gears, it is well known that, as the transverse contact ratio is raised to exceed 2, not only torque variations but also vibration and noise in operation are substantially reduced owing to the fact that they have at all times two teeth held in meshing contact and hence their teeth are subjected only to bending load of limited fluctuation.
With conventional W-N gears, however, it is not at all times advisable to raise the number of the contact tooth to 2 or above with a view to obtaining similar operational effects as it makes it necessary to accordingly increase the axial overlap contact ratio and this in turn necessitates a greater length of tooth trace as long as the helix angle is held within certain limits to maintain the tooth surface strength at a level desired, involving some problems to be considered in regard to machining and assembling accuracies, tooth deflection, etc..
One relatively recent prior art patent upon W-N gears is U.S. Pat. No. 3,855,874, issued Dec. 24, 1974. Such patent particularly relates to these W-N gears that include gear teeth having profiles each including circular addendum and dedendum arcs connected by a straight line segment at the pitch line area.